1. Field of the Invention
The present invention relates generally to light emitting devices, and in particular, to a light emitting device which can readily control the total quantity of light, under a balanced light emitting state of a plurality of light emitting elements. The present invention has particular applicability to a display device or an image reading device utilizing a plurality of light emitting diodes.
2. Description of the Background Art
Among many kinds of light emitting elements which are conventionally known, particularly a light emitting diode (LED) has been utilized in various electric appliances, with the recent development of the technique of manufacturing semiconductor devices. The present invention can be generally applied to a light emitting device having a plurality of light emitting elements, and as an example, the present invention is applied to a device which needs a plurality of light emitting diodes in the following description.
A light emitting diode is used, in one application, as a light source in discharging for a copying machine, a laser beam printer or the like. FIG. 1 is a view schematically showing a conventional copying machine. The copying machine shown in FIG. 1 is seen, for example, in Japanese Utility Model Publication, No. 2-10544. Referring to FIG. 1, light generated from a light source 22 illuminates an original 21, and the light reflected from the original 21 is applied through mirrors and a lens to a photosensitive drum 23. Toner responds to the applied photo signal and is deposited onto the surface of the photosensitive drum 23 with the aid of a magnetic brush 4. The toner deposited on the drum 23 is transferred onto a sheet of transfer paper 27, and a picture depicted on the original 21 is therefore defected onto the transfer paper 27. After the transfer, the photosensitive drum 23 receives light generated from a light source for discharging 28, and is thereby made free from a charge state caused by the reflected light. The toner deposited on the photosensitive drum 23 is removed by a rotary brush 29. It is pointed out that in the copying machine shown in FIG. 1, a light emitting diode is often utilized as the light source for discharging 28.
FIG. 2 is a view schematically showing an original reader in a conventional facsimile machine. The original reader shown in FIG. 2 is seen in "Nikkei Electronics" (No. 470, Apr. 3, 1989), p.159. Referring to FIG. 2, an original 30 is inserted into the reading device through the drum 31 or 32. Light is illuminated onto the original 30 from a light source 33 for reading, and the reflected light is applied to a CCD 38 through mirrors 34 to 36 and a lens 37. A video signal based on a picture depicted on the original 38 is therefore produced in the CCD 38. The video signal is transmitted to other facsimile machines through a transmitting apparatus (not shown). It is pointed out that in a facsimile machine as shown in FIG. 2, a light emitting diode is frequently used as the light source 33 for reading.
Additionally, a dot matrix display board, in which a plurality of light emitting diodes are utilized, is known. In the dot matrix display board, a plurality of light emitting diodes are arranged in a matrix, and information represented by characters or symbols is displayed by emission of light from selected light emitting diodes.
In the above electrical apparatus, a plurality of light emitting diodes are employed to form a necessary light source, and the following problems arise unless the quantity of light generated from each of the light emitting diodes is constant. If, for example, the light quantity of each of a plurality of light emitting diodes used as a light source for discharge in a copying machine is not constant, the discharge from the photosensitive drum 23 shown in FIG. 1 cannot be performed in a uniform manner. In addition, unless the light quantity of each of the light emitting diodes used in the light source 33 of the original reader shown in FIG. 2 is constant, an accurate video signal cannot be obtained in the CCD 38. Thus, the original cannot be read accurately in a copying machine or a facsimile machine. Additionally, if the light quantity of each of the light emitting diodes arranged in a dot matrix display board is not even, a display displayed on the dot matrix display board is difficult to read and an error can be caused in reading in an extreme case. It is pointed out that a driving current for each of the diodes should be controlled so that light generated from each of the diodes is even.
FIG. 3 is a circuit diagram showing a conventional driving circuit of a light source employing light emitting diodes. Referring to FIG. 3, the driving circuit includes light emitting diodes Dl to Dn each connected in parallel, resistors Rl to Rn connected to the diodes Dl to Dn, respectively, and a constant voltage source E. The resistors Rl to Rn are used to avoid the above described problems by controlling a driving current for each of the diodes Dl to Dn. Since the driving current/quantity of light characteristic of each of the light emitting diodes Dl to Dn is not constant, the quantity of light is made uniform by setting the values of the resistors Rl to Rn properly. The resistors Rl to Rn can be replaced with resistors Rxl to Rxk which are prepared in a factory and have various resistance values. A resistor having a proper value for each of the resistors Rl to Rn is connected whereby the quantity of light from each of the diodes Dl to Dn is made uniform. There exists a necessity for preparing many kinds of resistors having various resistance values in the factory.
Additionally, even once proper resistors Rl to Rn are connected, and the quantity of light from each of the light emitting diodes Dl to Dn is made uniform, the total quantity of light frequently should be controlled. In other words, not only balancing the quantity of light from the individual light emitting diodes Dl to Dn, but also the quantity of light from all the other diodes Dl to Dn should be controlled in some cases. In such a case, conventionally, all the resistors Rl to Rn are replaced with others and time required for the replacement is increase due to the increase of the number of the light emitting diodes Dl to Dn to be used. Furthermore, besides the above described replacement in the factory, the control for the total quantity of light is often necessary in the maintenance of the electrical instruments. Like other electrical components, the quantity of light from a light emitting diode tends to decrease according to an increase in use. In such a case, the resistors Rl to Rn have to be replaced in maintenance, in order to insure a constant total quantity of light. The time required for the maintenance is increased accordingly.